1. Industrial Fields of the Invention
The present invention relates to a method and apparatus for electrolytic treatment of less filterable sludge produced at a sewage and/or night soil treatment plant or the facility in order to modify same into an easily filterable type of sludge before it is subjected to filtration.
2. Prior Art and its Problems
Various types of sludge, such as raw sludge, digested sludge, excess sludge, and mixed sludge, produced in large quantities at a sewage and/or night soil treatment plant are first dewatered by means of a dehydrator, such as vacuum dehydrator, belt-type press, or filter press, and the dewatered sludge is then subjected to incineration or landfill disposal, or used for fertilization.
However, these types of sludge contain organic or inorganic particulate matter of highly hydrophillic nature and colloidal powder in large proportions, and further they have a large amount of organic matter dissolved or suspended therein; therefore they are very highly viscous and, insofar as they remain as such, it is very difficult to dehydrate them by aforesaid dehydrator.
As such, it is general practice in the prior art to add in and mix with the sludge prior to dehydration such inorganic flocculants as ferric chloride, green vitriol, and polyaluminum chloride, or such organopolymeric flocculants as polyacrylamide and formaldehyde condensates, or to further add and mix filter aids, such as slaked lime and diatomaceous earth, thereby adjusting and modifying the configuration of the sludge so as to facilitate dehydration by any such dehydrator as above mentioned. The modified sludge is then filtered and dewatered by means of the dehydrator.
As an alternative practice, it has also been known in the art to modify the sludge prior to dehydration by subjecting same to electrolytic treatment in an electrolytic tank (as disclosed in, for example, Japanese Patent Publication No. 16351/1979, and Japanese Patent Laid-Open Publication Nos. 105800/1981 and 129199/1987).
However, the first mentioned method of modifying the sludge prior to dehydration by adding chemical agents thereto involves various problems. One problem is that addition of large amounts of flocculants and filter aids into the sludge is inconvenient from the standpoint of filter maintenance. Another problem is that such addition results in increased cake production (e.g., where filter aids are used, the resulting cake production will be as high as 1.5 to 1.7 times), which involves greater trouble in cake incineration. Another problem is that both the filtrate and the filter cake may contain toxic residues, which require subsequent treatment for neutralization and toxicity elimination. A further problem is that the modified sludge still involves filtration difficulty and the resulting filter cake does not fully meet the required level of dewatering.
In contrast, the latter mentioned method of modifying the sludge prior to dewatering, i.e., the electrolytic treatment method, has an advantage that it requires no or little, if any, use of chemicals or filter aids and, therefore, it involves no such problem as above mentioned, insofar as it can be carried out satisfactorily or effectively.
This method of modifying the sludge by electrolytic treatment is an application of the principle that when electricity is applied to a concentrated sludge in an electrolytic tank, ions in the sludge are caused to migrate so that less filterable suspended matter and dissolved substances in the sludge are electrically decomposed, there being thus repetition of oxidation, reduction, neutralization, and deposition stages with respect to the so decomposed matter, with the result that flocs in the sludge are rendered more hydrophobic than in the case where aforesaid chemical agents are used so that the flocs are modified into easy-to-filter flocs. However, known methods and apparatuses for sludge modification by electrolytic treatment have problems yet to be solved as discussed below. Attempts to solve those problems have not yet gone beyond the stage of experiment, and in reality there is as yet no apparatus based on such attempt that has been put in practical application at any sewage treatment plant or elsewhere.
According to series of experiments conducted, with the prior art method for sludge modification by electrolytic treatment, where the positive and negative electrode plates arranged in the electrolytic tank are spaced as narrowly as possible relative to one another, higher electrolysis or modification efficiency is obtainable, but if the distance between adjacent electrode plates is excessively small, gas bubbles, such as oxygen and hydrogen, generated from individual electrode plates and hardened flocs as a result of electrolysis of the sludge are likely to be retained between the electrode plates. When such bubbles and/or inclusions, e.g., hardened flocs, are retained between electrode plates, the trouble is that they interfere with electric current conduction, so that the electrolysis or modification efficiency will be lowered to the extent that the electrolytic treatment method for sludge modification is far from being put to practical use. However, if the inter-electrode space is made wider, a decrease in current density is inevitable and electrolytic operation requires considerable time, though gas bubbles and inclusions produced between individual electrode plates can be prevented from being retained between and depositing on the electrode plates; in this case, too, therefore, it is inevitable that the electrolysis or modification efficiency is lowered to the extent that the sludge modification method can hardly be put to practical use.
Techniques intended to solve aforesaid problems in the case where space intervals between individual electrodes are reduced, that is, methods and apparatuses for removal of inclusions produced between the electrode plates, are described in the following publications, for example. Japanese Patent Publication No. 16351/1979 discloses a method and apparatus wherein a solution (of caustic soda) capable of eluting inclusions formed between surfaces of individual electrode plates in an electrolytic tank, which solution is contained in an eluting solution tank, is intermittently circulated between the electrolytic tank and the eluting solution tank, whereby the inclusions formed between the electrode plates can be removed. Japanese Patent Laid Open Publication No. 129199/1987 discloses a method and apparatus wherein sludge in an electrolytic tank is agitated by an agitation blade provided below electrode plates in the electrolytic tank so that a downcurrent is produced of the sludge, a portion of the sludge being drawn from the bottom of the tank, the so drawn sludge being sprinkled into the tank from a level above the electrode plates so that a downcurrent of sludge is produced which flows downward between individual electrode plates arranged in side by side relation and along the surfaces of the electrode plates, so that inclusions formed between individual electrode plates can be carried away by a combination of the first and second mentioned downcurrents.
However, the first mentioned method and apparatus wherein the eluting solution is circulated between the electrolytic tank and the eluting solution tank is not directed to removal of inclusions in the course of electrolytic sludge treatment. As such, the difficulty with this technique is that operation has to be interrupted in the course of electrolytic sludge treatment. The second mentioned method and apparatus wherein downcurrents of sludge are produced in the electrolytic tank is such that the sludge is caused to flow downward against the upward orienting force of gas bubbles produced from the electrode plates. Therefore, this latter technique involves a problem that if the sludge is vigorously agitated by the agitation blade in order to provide a uniform and sufficient downcurrent flowing between individual electrode plates, flocs of modified sludge are broken down by the agitation blade, and another problem that unless a uniform and sufficient downcurrent is produced between individual electrode plates, gas bubbles and inclusions formed between the electrode plates cannot smoothly be removed.